ADHD Can you be tested privately?

My son has ASD along with other co-occuring specific learning difficulties. We've long suspected he also suffers with ADHD, however, we've never discussed it with him and didn't have him tested because he reacted badly to his ASD diagnosis and we didn't want to cause him further distress.

Recently, he's come to me and asked if he can be assessed for ADHD. I've looked on the NHS website and diagnosis for adults seems less common. He's 16 now and does; and always has, struggled with his concentration greatly. He says he doesn't want to take pills for his symptoms, but i'm not sure if this is the only treatment for the condition.

I've questioned what he would hope to gain from an assessment, but he doesn't seem to know.

I'm really not familiar with the full symptoms of ADHD and have chalked most of his behaviours up to his ASD. Can anyone tell me is ADHD a common thing for those on the spectrum. Is it possible to assess privately as a young adult and if he does have a diagnosis are their methods other than pills to treat the condition? 

Many Thanks

Coogybear